1. Field of the Invention
This invention relates to a heat fixing method in which a toner image on a transfer material formed by a heat meltable toner is heat fixed.
2. Related Background Art
In the prior art, for the fixing device to be used in the heat fixing method, there have been frequently used the roller fixing systems which perform heating and pressurization of, while conveying, a transfer material having an unfixed toner image sandwiched between the heating roller maintained at a predetermined temperature and a pressurizing roller pressure contacted with the heating roller. However, in the device of this kind, for prevention of the phenomenon of transfer of toner to the heating roller (so called off-set phenomenon), the heating roller is required to be maintained at an optimum temperature, and further the heat capacity of the heating member for heating the heating roller must be made large. If the heat capacity of the heating member is small, the heat capacity of the heating roller becomes small, and in that case, from the relationship with the heat content supplied by the heating member, the temperature of heating roller is susceptible to great changes by paper passage or other external factors. When that temperature is changed toward the lower temperature side, fixing failure or low temperature off-set occurs due to shortage of softening melting of the toner, while when changed toward the higher temperature side, the toner will be completely melted, whereby high temperature off-set occurs due to lowering in agglomeration force of the toner. If the heat capacity of the heating member is made larger for avoiding such problems, the time for elevating the temperature of the heating roll to a predetermined temperature becomes longer, whereby there ensues the problem that the waiting time becomes longer in using the fixing device. U.S. Pat. No. 3,578,797 proposes a method of fixing without occurrence of off-set by use of a heating member, by heat melting a toner image, then cooling the toner image to make it under a relatively higher viscosity state, followed by peel-off of the transfer material having the toner image from the heating member web under the state where the tendency of attachment of toner is weakened. However, since this method employs the method of heating without pressure contacting the toner image and the transfer material against the heating member, the heat transmission efficiency between the heating member and the toner image becomes poorer, whereby enormous energy is required for fixing.
Japanese Patent Publication No. 51-29825 proposes a method of heat melting a toner image within a short time by effecting improvement of heat transmission efficiency by pressure contacting the heating member with the toner image. However, this method employs a system in which heating is effected under the state where the toner image and the transfer material are previously sandwiched under pressure between a pair of heating members, and thereafter cooling is effected compulsorily. Specifically, the toner image is heated with a pair of heating members from both front and back surfaces, and therefore it appears that such method is efficient in aspect of energy. Practically, however, energy efficiency is consequently poor, for such reasons that the toner image is required to be sufficiently heated from the transfer material side, and further that a compulsory cooling means is required because the toner image cannot be peeled off unless the transfer material once heated is abruptly cooled in the subsequent cooling step. Further, since a heating member with relatively larger heat capacity is used, heat dissipation into the machine is increased, whereby there was also the problem that unnecessary temperature elevation within the machine was brought about.